In a constant velocity joint built in a driving system of an automobile, a shaft insertion hole is provided in an inner ring, and an axially extending male spline that is formed on an outer circumference of an end portion of a shaft and an axially extending female spline that is formed on an inner circumference of the insertion hole are caused to fit onto each other. And a retaining ring as a retaining member is used to restrict the axial movement of the shaft with respect to the inner ring.
For a concrete construction of a shaft mounting structure of a constant velocity joint, for example, as shown in FIG. 4, a shaft 101 is inserted into an inner ring 100, and a retaining ring 102 is attached to a leading end of the shaft 101 that protrudes from the inner ring 100. And the retaining ring 102 is caused to interfere at the back of the side opposite to the shaft insertion side of the inner ring 100, whereby the axial movement of the shaft 101 is restricted (refer to Japanese Patent Laid-Open No. 2006-218983). Also as shown in FIG. 5, there is a case where a retaining ring 102 and an inner ring 100 are caused to interfere with each other at the front of the shaft insertion side (refer to US 2005/0124423A1).
Incidentally, particularly, in the case of a closed type in which an end of an outer ring of a constant velocity joint is closed and the construction is such that a retaining ring interferes with an inner ring on the back side as shown in FIG. 4, it is impossible to attach the retaining ring, with the inner ring and the outer ring combined with each other. Furthermore, in this case, after the combining of the inner ring and the outer ring, it is impossible to visually recognize the attachment state of the retaining ring and the like from the outside and hence it is difficult to check quality. On the other hand, in a structure in which the retaining ring and the inner ring interfere with each other on the front side as shown in FIG. 5, this defect does not exist and the structure of FIG. 5 is desirable in terms of the ease of assembling work and the visibility of the assembled state.
In a conventional example as shown in FIG. 5, however, a groove 103 to which the retaining ring 102 is attached is formed in a male spline 101a of the shaft 101. Therefore, stress concentration occurs in the place of this groove 103, posing the problem of low strength.